Problem: Solve for $r$ : $-15 = -18 + r$
Solution: Add $18$ to both sides: $ \begin{eqnarray} \\ \hphantom{-15 {+ 18}} &\hphantom{=}& \hphantom{r} \\ -15 &=& -18 + r \\ \\ {+18} && {+18} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -15 {+ 18} &=& r \\ \end{eqnarray} $ Simplify. $ r = 3$